1. Field of the Invention
This invention relates to a print storage medium in which an electric signal is converted into thermal energy and ink is then transferred onto a transfer material with the aid of this thermal energy, thereby effecting print-storage.
2. Description of Related Art
There are various conventional print-storage devices in which electric energy is converted into thermal energy. One such device includes a thermal storage medium provided with either a heat-softening ink layer or a heat-sublimating ink layer on the back surface of a base sheet of thermal sensitive paper. The front surface of the base sheet is provided with a heat sensitive color developing layer that is depressed against a heating-head. The heating-head generates heat in response to a picture signal for softening an ink layer formed on the back surface of the base sheet for transfer onto a print-storage transfer material. (See, for example, Japanese Patent Preliminary Publication No. 53-84735.)
In a second prior art print-storage device, an electric picture signal is applied to a support element through a needle electrode. Current flows through the support element to the ink layer of a print-storage medium, generating heat and melting the ink. The melted ink is then transferred onto a transfer material. The support element is typically in the form of a ribbon made of dispersed metal powder and resin but can also be made of an electrically conductive film having a high resistance. (Gazo Denshi Gakkai Publication. 1982. Vol. 11 No. 1, paper No. 17 of 12th National Conference)
A third print-storage device is shown in FIG. 1, which depicts a thermal print-storage medium having an upper layer 30 of low resistance, a lower layer 31 of high resistance, an electrically conductive layer 32, and an ink layer 33. These layers are placed one over the other to form a print-storage medium 34. Needle electrode 35 and return-current electrode 36 are in contact with the surface of the upper layer 30 of print-storage medium 34. A current is run through needle electrode 35, upper layer 30, lower layer 31, conductor 32, and return electrode 36 to generate heat. The heat generated causes the ink layer 33 to melt and the melted ink is then transferred onto a transfer material. (Japanese Patent Preliminary Publication No. 56-93585) However, these prior art devices have several drawbacks.
The first prior art device outlined above, is a type in which heat generated by the heating head is transferred from the thermal light-emitting layer to the ink layer through the base sheet. This causes the ink layer to melt for printing. One drawback with this device is that conductive heat transfer requires a long time, and therefore reduces printing speed. Also, the heat generated by the heating head is diffused during heat transfer, reducing the amount of heat energy conducted to the ink layer. Therefore, the type material usable as the ink layer is severely restrained, and dot modulation (wherein the printed dot size is varied in response to the amount of heat energy applied) is virtually impossible.
The second prior art device outlined above is a type in which the ink layer is electrically conductive to generate heat. One drawback with this method is that the ink layer is made conductive by adding conductive materials. This makes it difficult to control the color tone of the ink layer. Further, this device causes a wider area of the ink layer to be heated due to heat conduction which leads to poor dot printing precision. In addition, electrical anisotropy of the ink-supporting material is inadequate, causing current to flow in areas other than those intended. Leakage within the ink supporting material also causes a large amount of energy loss.
The third prior art device outlined above has a drawback similar to that of the second method. Upper layer 30 does not exhibit sufficient electrical anisotropy, causing current to flow through areas other than those intended. Thus, leakage occurs within the upper layer 30, leading to a large amount of energy loss. In this method, a needle electrode 35 is caused to contact upper surface 30 of low resistance as shown in FIG. 1. Needle electrode 35 contacts upper layer 30 at a "point." Since the contact area is usually much smaller than the cross-sectional area of needle electrode 35, the contact resistance between the needle electrode 35 and the upper layer 30 is high. The current flowing through the upper layer 30 is concentrated as shown in FIG. 2. Upper layer 30 disturbs the uniform flow of current leading to a nonuniform generation of heat. In addition, upper layer 30 can be damaged by excess current flow through the contact point.
Further, since the current through upper layer 30 is concentrated at the contact point, the amount of heat cannot be easily controlled by varying the amount of current through the needle electrode 30. This causes improper gradation within a picture. Since current flows from the needle electrode 35 to the lower layer 36 through the upper layer 30, and then flows to the return current electrode 36 through the lower layer 31, current passes through the lower layer 31 twice. Therefore, it is possible for heat to be generated at two locations within the lower layer 31, causing poor print energy efficiency. In addition, it is possible for two dots to be printed rather than one, impairing clear print storage.
An object of the present invention is to provide a print storage medium that overcomes the disadvantages of the prior art devices by providing a needle electrode and a print storage medium having low electric conduction loss.
Further, objects of the present invention are a print storage medium having high printed dot resolution, high energy efficiency, and good durability.